In the field of electrochemistry there is a well-known electrochemical cell known as a chlor-alkali cell. In this cell, an electric current is passed through a saturated brine (sodium chloride salt) solution to produce chlorine gas and caustic soda (sodium hydroxide). A large portion of the chlorine and caustic soda for the chemical and plastics industries are produced in chlor-alkali cells.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available NAFION manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a precious metal coating to yield what is known in the art as a dimensionally stable anode.
One of the unwanted byproducts present in a chlor-alkali cell is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
It has been estimated that 25 percent of the electrical energy required to operate a chlor-alkali cell is utilized due to the formation of hydrogen at the cathode. Hence, the prevention of hydrogen formation, e.g., by oxygen reduction at the cathode, can lead to substantial savings in the cost of electricity required to operate the cell. In fairly recent attempts to achieve cost savings and energy savings in respect of operating chlor-alkali cells, attention has been directed to various forms of what are known as oxygen (air) cathodes. These cathodes prevent the formation of molecular hydrogen at the cathode and instead reduce oxygen to form hydroxyl ions. Savings in cost for electrical energy are thereby achieved.
Typical operation of an oxygen (air) cathode requires use of air pressure below bubble-through and an oxygen flow rate sufficient to provide no less than about 2.5 times the stoichiometric amount required for reaction. Customarily pressure is measured in inches of water corresponding to the air pressure delivered. Operation of an oxygen cathode in the normal mode involves use of conditions which avoid blow-through of the oxygen or air because it is generally observed that performance decreases at air pressures incipient to blow-through. Also, there are problems with handling air which has made its way into the catholyte chamber in cells designed for standard operation. These ideas have served as roadblocks to any thought that blow-through may be preferable in operation of gas depolarized electrodes. Pressures in excess of blow-through do result in better voltage. The term "blow-through" pressures as used herein means oxygen or air pressures exceeding the pressure(s) at which bubble-through of such gas occurs resulting in reduced operating cathode potential. Usually oxygen/air pressures of about 5 to about 15 psig are satisfactory to accomplish blow-through. The term "bubble-through" as used herein means that air supplied to the cathode penetrates the cathode and appears in the electrolyte as a heterogeneous phase of bubbles.
U.S. Pat. No. 4,221,644 to Ronald L. LaBarre is directed to a method for operating oxygen electrodes with the stated purpose of maximizing the power efficiency available while minimizing the voltage necessary to operate such oxygen electrodes. The LaBarre method includes control of the pressure of the air feed side of the oxygen electrode, control of the total flow of the air feed side, humidification of the air feed side of the oxygen electrode and eliminating CO.sub.2 from the air feed to increase the lifetime of such electrodes as applied to a chlor-alkali electrolytic cell. At column 4, lines 9-14 of LaBarre, it is stated that the air is fed to the interior of the oxygen compartment at a positive gauge pressure so as to accomplish a total flow rate in excess of the theoretical stoichiometric amount of oxygen necessary for the reaction. At column 4, lines 47-50, LaBarre states that the CO.sub.2 -free air is humidified and provided at a positive gauge pressure and at a positive total flow of from 1.5 to 10 times the stoichiometric amount of oxygen. At column 10 of LaBarre, it is stated that the pressure in the oxygen compartment 24 is higher than that in the cathode compartment 22. The increased pressure, which may be zero gauge to bubble-through but due to the electrolyte head may be negative absolute, assists in mass transfer of the oxidizing gas such as air with CO.sub.2 removed into the cathode 18 thereby preventing oxygen depletion in the reaction zone within the cathode 18 and leading to a longer cathode lifetime. It is stated that this pressure differential is based upon the partial pressure of the oxygen present. The preferred total flows are between 0 and 10 times the theoretical stoichiometric amount of oxygen necessary for the reaction with a flow of about 2.5 times being stated as the best.
The present "blow through" oxygen (air) pressure method of operating an oxygen (air) cathode differs from that of U.S. Pat. No. 4,221,644 in the following respects. Whereas in the LaBarre patent, feed air is introduced at one end of the cathode chamber and depleted air withdrawn from the opposite end containing all the nitrogen of the feed streams; the present method introduces the feed air in one and then removes all or some of the depleted air from the electrolyte side of the cathode. In the LaBarre patent, the nitrogen must diffuse back out of the interior of the cathode, and the oxygen must diffuse in through a stagnant layer of nitrogen. In the present method, the nitrogen and oxygen are physically flowed through the porous cathode face; and whereas diffusion must occur in the interior of the active cathode area, the diffusion path is shortened and voltage is improved.